Process for treating plastics with alkaline permanganate solutions

ABSTRACT

An improved alkaline permanganate composition for etching printed circuit boards is provided by incorporating a secondary oxidant in the solution capable of oxidizing manganate ion to permanganate ion.

This is a divisional of co-pending application Ser. No. 618,281 filed onJune 7, 1984 now U.S. Pat No. 4,592,852.

BACKGROUND OF THE INVENTION

The invention relates to the metal plating of plastics and, inparticular, to enhancing the adhesion of metal plating to the plasticsubstrate of electronic circuit boards by treating the board with analkaline permanganate solution prior to electroless metal plating of theboard.

The metal plating of plastic parts is well-known to be of considerablecommercial importance because the desirable characteristics of both theplastic and the metal are combined to offer the technical and aestheticadvantages of each. Thus, a part plated with a bright, metallic finishtakes advantage of the economies in cost and weight afforded bysubstituting molded plastic parts for metal and, additionally, theplated finishes are not as susceptible to pitting and corrosion becausethere is no galvanic reaction between a plastic substrate and a platedmetal.

An important process is the preparation of electronic circuit boardswhich requires the elctroless plating of a conductive metal layer,usually copper, onto the plastic substrate of the board, such as epoxy,and for convenience the following description will relate specificallyto this process.

These boards vary in design and may have a copper layer on each surfaceof the epoxy (two-sided boards) or they can be multi-layer boards whichhave a plurality of inter-leaved parallel planar copper and epoxylayers. In both type boards throughholes are drilled in the board andmetal plated to facilitate connection between the circuits on the copperlayers. The throughholes present an additional plating problem becauseresin smear on the exposed copper caused by the drilling operation actsas an insulator between the metal of the through-hole and copper layerand must be removed prior to plating. The smear is usually removed usingacid and this process degrades the physical integrity of the hole makingit difficult to metallize and provides little or no adhesive support forthe metallic deposit.

The problems in plating either the through-holes or other plastic partsof the board are well-known in the art and a number of methods have beendeveloped to improve the adhesion of the metal plating to the epoxy.These methods generally employ oxidants to etch the surface of theplastic prior to plating and include chromic acid, sulfuric acid andacidic and alkaline permanganate solutions. The toxicity of the chromiumcompounds and their potential hazards as water pollutants and the safetyprecautions needed with sulfuric acid have increased the commercial useof permanganate solutions, particularly alkaline permanganate solutions,and a number of patents have been granted in this area.

U.S. Pat. No. 3,652,351 shows the etching ofacrylonitrile-butadiene-styrene interpolymers (ABS plastics) using acomposition containing manganate and hydroxyl ions. U.S. Pat. Nos.4,042,729 and 4,054,693 disclose stable, highly active etching solutionscontaining particular ratios of manganate ions and permanganate ions bycontrolling the pH in the range of 11 to 13. U.S. Pat. No. 4,425,380 isspecifically directed to cleaning through-holes of residual manganeseprior to plating by contacting the etched plastic with a water solublecompound oxidizable by permanganate to reduce the manganese residues toa low oxidation state, e.g., SnCl₂ -HCl, formaldehyde, followed bycontacting with hot alkaline hydroxide. The disclosures of the abovepatents are hereby incorporated by reference.

The present invention relates to treating the plastic surface beforeplating and particularly to etching the plastic with an alkalinepermanganate solution to remove smear and to improve the adhesiveness ofthe metal plate to the plastic board. U.S. Pat. No. 4,042,729, supra,attempted to improve the use of alkaline permanganate solutions to treatplastic circuit boards and notes the many problems associated with thesesolutions. Thus, acidic permanganate solutions are notoriously unstable,have a short life and rapidly decompose to manganese dioxide. Alkalinepermanganate solutions present similar problems but, according to theinvention, are highly active, easily controllable and stable providedthe molar ratio of manganate ion to permanganate ion is up to about 1.2and the pH of the solution is controlled in the range of 11 to 13 by theuse of buffers or pH adjustors. The patent also provides a means forrejuvenating the bath to remove organics and proudce permanganate bydisproportionation of manganate comprising bubbling CO₂ into the bath tolower the pH from 12.5 to 11 to 11.5, heating to form permanganate ionsand manganese dioxide from the manganate ions, cooling, precipitatingcarbonates and filtering to remove the manganese dioxide and carbonates.The preparation and control of the compositions and rejuvenatingprocedures of this patent are unwieldly and time consuming and there isa need for improved alkaline permanganate etching solutions and methodsfor using the solutions to etch plastic substrates.

SUMMARY OF THE INVENTION

It has now been discovered that alkaline permanganate etchingcompositions comprising permanganate ions and a secondary oxidantcapable of oxidizing manganate ions to permanganate ions are highlyeffective to treat plastic substrates prior to electroless metaldeposition of the plastic. The improved compostions when used to treatplastics such as epoxy and other resins exhibit excellent stability andlong term operating life. The formation of undesirable lower oxidationstate manganese products such as manganese dioxide, is lowered andadditionally, the activity of the solution is also unexpectedly enhancedand shorter treatment times are generally required than with prior artcompositions. It is another important unexpected feature of theinvention that the improved alkaline permanganate etching processproceeds without any significant adverse operating effects which would,for example, increase the number of treated parts being rejected forfailing to meet specifications. While no theory can be advanced for theunexpected results achieved when using the compositions of the inventionit remains that the invention provides an etchant composition andprocedure for preparing plastic substrates for metallization superior toany known to the prior art.

The alkaline permanganate solutions are well-known in the art andsuitable compositions are shown in U.S. Pat. Nos. 4,042,729 and4,425,380. In general, the permanganate component is present in anamount of about 1 gram per liter (g/l) to the limit of its solubility inthe medium and the pH of the composition is alkaline and preferablyabove about 10, more preferably above about 13, e.g., between 13 and 14.The amount of the secondary oxidant component having an oxidationpotential greater than the oxidation potential of the alkalinepermanganate solution may vary widely from about 0.5 gram equivalent ofoxidant per gram equivalent of the manganate ion concentration to astoichiometric amount, or more.

The composition is employed at an elevated temperature and is, ingeneral, about 90° F. to the lesser of the boiling point of the solutionand the softening point of the plastic. For epoxy boards the temperatureis preferably about 140° to 160° F.

The effective contact time will vary with the concentration andtemperature of the solution and the plastic substrate being treated andin general will not exceed about 30 minutes, preferably 5 to 15 minutes,e.g., 10 minutes. For epoxy boards a contact time of about 8-12 minutesat a temperature of about 145°-155° F. has been found to provideexcellent results.

In general and for purposes of illustration, the process of using thecompositions of the invention in manufacturing printed circuit boardsand, in particular, boards containing through-holes, is a sequence ofsteps commencing with a commercially available laminate ormulti-laminate made from, e.g., paper-epoxy or glass-epoxy material. Apredesigned series of through-holes is formed in the board in anyconventional manner, then preferably etched with an acid to remove anyepoxy smear over the exposed metal laminate surface and to etch-back theepoxy layer, and water rinsed. This may be followed by contacting theboard with a swellant type composition to enhance the effect of thesubsequent permanganate etching step as described in U.S. Pat. Nos.3,758,732; 3,865,623 and 4,086,128, the disclosures of which are herebyincorporated by reference. After water rinsing, the board is etched withthe alkaline permanganate solution of the invention and water rinsed.Neutralization with a reductant to solubilize manganese residues bylowering their oxidation state is then performed and the board rinsedproviding a board having all the copper layers in the throughholesexposed and the epoxy etched ready for electroless metal plating usingconventional procedures. A preferred step is to now dissolve some of theglass fibers in the epoxy using, for example, acid fluorides, and topretreat the board by cleaning with a composition such as ENPLATE®PC-475 to remove hydrocarbon soils and oxides and micro-etching of thecopper using an etchant such as ENPLATE AD-485. Thereafter the board isimmersed in a catalyst, such as a tin-palladium solution, whichconditions the surface of the epoxy for electroless copper plating.ENPLATE Activator 444 sold by Enthone, Incorporated is exemplary of thistype catalyst. Following a water rinse, the laminate is immersed in anaccelerator such as ENPLATE PA-491 to activate the catalyst by freeingthe metal palladium ions on the board. After a water rinse, the board isdried and is immersed in an electroless copper plating solution for aperiod of time sufficient to plate copper to the desired thickness onthe surfaces and to plate the surfaces of the holes to form through-holeconnections between the laminate surfaces. ENPLATE CU-700 and othersimilar plating compositions may be employed. The boards may then beelectroplated using conventional techniques if a thicker coating isdesired.

DETAILED DESCRIPTION OF THE INVENTION

The compositions of the invention have been found to be particularlyuseful for treating epoxy resins although other suitable polymericresins may also be conditioned. The electronic circuit boards arecommercially available and are of known structure and composition andinclude paper-epoxy and glass-epoxy laminates. In general, an epoxyresin means a polymeric compound containing at least two epoxy groups inone molecule and synthetic resins formed by ring-opening reactions ofthe epoxy groups of the compounds. An epoxy resin extensively used inthe printed circuit industry is a cured epoxy resulting from acondensation product of epichlorohydrin and bisphenol A. A board termedFR-4, which is a fire retardant epoxy resin fiber glass cloth laminate,has demonstrated to be effectively treated by the process of theinvention.

The epoxy resin may contain glass fibers, paper, synthetic fibers,carbon black, alumina powders, silica powders, wax, etc. as fillers,pigments, mold release agents, reinforcing agents, etc., or can be usedtogether with phenol resins, urea resins, melamine resins, and the like.

The alkaline pH of the composition may be provided by any suitablesource and is preferably an alkaline hydroxide such as lithium, sodium,potassium, cesium, and tetraalkyl ammonium. Sodium hydroxide andpotassium hydroxide are preferred. The amount of hydroxyl ions can varywidely and is preferably about 10 to 150 g/l, or higher, and morepreferably about 20 to 90 g/l, most preferably 30-70 g/l. It ispreferred to measure the hydroxide ion concentration at predeterminedintervals and to add more alkali as needed to maintain the desiredconcentration.

Any source of permanganate ion may be employed which is stable andsoluble in the solution. It is preferred to employ alkali metal oralkaline earth metal salts such as sodium, potassium, lithium, cesium,calcium, etc., with potassium permanganate being preferred because ofits availability and demonstrated effectiveness.

The amount of permanganate salt used in the solution may vary widelyfrom about 1 g/l up to the limit of solubility in the medium. Apreferred range is about 10 g/l to 100 g/l and more preferably about 40g/l to 90 g/l, e.g., 50-70 g/l.

The other essential component of the composition is a material having anoxidation potential higher than the oxidation potential of thepermanganate solution and which is capable of oxidizing manganate ionsto permanganate ions. While any such oxidant may be used, it ispreferred that the oxidant have an oxidation potential greater thanabout 10% higher than the oxidation potential of the permanganatesolution, and more preferably greater than about 25%, e.g., 50-100%, orhigher. Exemplary oxidants include chlorine, bromine, ozone,hypochlorite salts, metaperiodate salts, trichloro-s-triazinetrione andits salts, and the like. Sodium hypochlorite is preferred because of itsease of use, availability, low cost and demonstrated effectiveness.Commercially available 13% by weight sodium hypochlorite solution (15%available chlorine) has provided excellent results.

The amount of oxidant employed may vary widely and, in a preferredembodiment, is correlated to the manganate ion concentration of thepermanganate solution and, most preferably, to the total manganate ionplus permanganate ion concentration. In general, for a hypochloritesalt, a range of about 1 g/l to 100 g/l or more, may be employed,preferably 5 g/l to 50 g/l, e.g., 5 g/l to 25 g/l.

It is an important feature of the invention that permanganate ions bepresent at the desired concentration in order to maintain the activityof the solution. As is well-known in the art, manganate ion is formedduring the treatment process which reduces the activity of the solutionand leads to other undesirable results such as manganese dioxide sludgeformation. The use of the secondary oxidant in the composition ineffective oxidizing amounts maintains the activity of the solution,stabilizes the solution, consumes less of the expensive permanganatesalt while consuming a secondary oxidant which, in many cases, is muchless expensive than the permanganate salt, among other benefits.

While different methods can be employed to practice the invention, it ispreferred that at desired intervals during the process the concentrationof permanganate and manganate ions be determined by analyticaltechniques. The ratio of permanganate ion concentration to the sum ofthe permanganate ion and manganate ion concentrations is calculated witha ratio greater than about 0.5 being desired, and increasing ratiosproviding, in general, a preferred process. A ratio greater than about0.7 is preferred, with ratios greater than about 0.8 and 0.9 being mosthighly preferred. Depending on the ratio, an effective amount of thesecondary oxidant is added, as needed, to the solution to control theratio at the desired level. Amounts of oxidant will vary and can bereadily determined by those skilled in the art. Preferably, the ratio isdetermined, and, if, for example, is less than 0.7, 3% by volume of a13% by weight NaOCl solution is added to the bath. After about 1 hourthe ratio is again determined and should be above the operating level of0.7. Since higher ratios are preferred, if the ratio is say between 0.8and 0.85, it has been found effective to add 1% by volume of the NaOClsolution to the bath. This will maintain the ratio, depending on usageof the bath, for an operating period of at least a day (24 hours). It ispreferred to analyze the bath on a daily basis and to make adjustmentsas needed as will be appreciated by those skilled in the art.

It is another important feature of the invention that the permanganateion concentration be also maintained at a predetermined level in thesolution, e.g., its original concentration. Permanganate ions may beadded to the solution to bring the solution to the desired permanganateion concentration based on the determined permanganate ionconcentration. It has been found that highly satisfactory operatingresults have been achieved by summing the permanganate ion and manganateion concentrations and adding permanganate ions based on this value andthis procedure is preferred based on its demonstrated effectiveness.Thus, based on potassium permanganate, if a permanganate concentrationof 65 g/l is desired and the sum of the potassium permanganate andpotassium manganate concentration is 55 g/l, 10 g/l of potassiumpermanganate would be added to the solution.

Other additives, such as wetting agents, may be used in the compositionfor special purposes as is known in the art.

The composition media is preferably aqueous and substantially inert withrespect to the plastic being treated. Other media may also be employedsuch as alcohols with water being preferred for economic reasons and forits demonstrated effectiveness.

To practice the method of the invention the plastic substrate iscontacted with the composition at an elevated temperature for a timesufficient to render the surface receptive to the electroless metalplating process. Contacting procedures may vary widely and satisfactoryresults for epoxy resin are provided by immersing the part in thesolvent for between approximately 5 to 15 minutes at 140° to 160° F. Thetime and temperature will vary depending on the substrate being treatedand the composition of the solution as will be appreciated by thoseskilled in the art. Other means such as spraying, may be used fortreating the plastic part.

The treated plastic part is then rinsed to remove excess solution andmanganese residues removed by neutralization or chemical reduction usingreductants such as hydrazine and oxalic acid.

The etched plastic part is now prepared for electroless metal plating byknown means as described in U.S. Pat. Nos. 2,874,072; 3,011,920;3,075,855; 3,095,309; 3,672,938 and 3,736,156; the disclosures of saidpatents being hereby incorporated by reference. In general, the plasticsurface is catalyzed employing a solution containing tin and palladiumfollowed by treatment with an accelerator solution, usually an acid, toremove excess tin and provide a palladium rich surface. The plastic isnow ready for plating with an electroless metal bath such as copper.Other methods of deposition may also be used such as vacuum vapordeposition, electrolytic plating or a combination of electroless platingand electrolytic plating.

The present invention will now be described in detail by reference tothe following examples.

EXAMPLE I

The following example illustrates the use of the composition to providean efficient, effective method for the electroless metal plating of aprinted circuit board.

A two-sided copper clad epoxy-glass FR-4 laminate board was metallizedusing the following procedure:

(a) strip the copper clad from the surface of the board with 50% HNO₃ atroom temperature;

(b) rinse in water;

(c) clean the surface with 96% sulfuric acid for 45 seconds at roomtemperature;

(d) rinse in water for 5 minutes;

(e) immerse for 5 minutes at 140° F. with mild agitation in a solutioncomprising:

Sodium hydroxide: 90 g

N-Methyl-2-pyrrolidone: 100 ml

Ethyleneglycol monobutyl ether: 20 ml

Water (to make): 1000 ml

(f) rinse for 5 minutes in water;

(g) etch for 10 minutes at 150° F. with mild agitation in a solutioncomprising:

KMnO₄ : 65 g

NaOH: 50 g

NaOCl: 10 g

Water (to make): 1000 ml

(h) rinse for 5 minutes in water;

(i) neutralize for 5 minutes at 140° F. in a solution comprising:

Hydrazine . H₂ SO₄ : 25 g

HCl (37% aqueous): 30 ml

Water (to make): 1000 ml

(j) rinse in water for 3 minutes;

(k) immerse in a conditioning cleaner such as ENPLATE® PC-475 for 5minutes at 145° F. (ENPLATE PC-475 manufactured by Enthone,Incorporated, West Haven, Conn.);

(l) rinse in water for 3 minutes;

(m) immerse in a one-step palladium catalyst such as ENPLATE Activator444 for 5 minutes at 75° F.;

(n) rinse with water;

(o) immerse in a post activator solution such as ENPLATE PA-491 for 5minutes at room temperature;

(p) rinse in water for 2 minutes;

(q) metallize in an electroless copper solution such as ENPLATE CU-700for 30 minutes at 118° F.;

(r) rinse with water and air dry.

The metallized layer was tested for adhesion by firmly attaching a stripof adhesive tape to the metallized surface and then removing the tapewith a quick snapping motion. There was no transfer of the metallizeddeposit to the adhesive backing of the tape.

EXAMPLE II

The following example illustrates the use of the composition to providean efficient, effective method for treating a printed circuit board.

A multi-layer FR-4 epoxy fiberglass copper clad board having drilledthrough-holes was metallized using the following procedure:

(a) immerse the board for 5 minutes at 140° F. in a solution comprising:

N-methyl-2-pyrrolidone: 50 ml

Ethyleneglycol monobutyl ether: 10 ml

Sodium hydroxide: 45 g

Water (to make): 500 ml

(b) rinse in water;

(c) immerse the board for 10 minutes at 150° F. in a solutioncomprising:

Potassium permanganate: 30 g

Sodium hydroxide: 25 g

Sodium hypochlorite: 5 g

Water (to make): 500 ml

(d) rinse in water;

(e) determine the permanganate and manganate concentrations and sodiumhydroxide concentration of the solution of step (c) at predeterminedintervals;

(f) calculate the ratio of potassium permanganate concentration to thesum of the potassium permanganate and potassium manganate concentration;

(g) add potassium permanganate and sodium hydroxide as needed tomaintain their original concentrations the amount of KMnO₄ being addedbased on the original KMnO₄ concentration minus the sum of the KMnO₄ andK₂ MnO₄ concentration);

(h) add sodium hypochlorite to maintain the ratio above 0.8;

(i) repeat steps (a)-(h) until a loading of 90 ft² panel/gallonsolution.

The board when metallized by steps (i)-(r) of EXAMPLE I producedstrongly bonded metal deposits.

The permanganate solution required 11.8 g KMnO₄ to maintain the originalpermanganate concentration and 29 g NaOCl to maintain a ratio of 0.8 orbetter. The permanganate solution had no trace of insoluble matter onthe bottom of the container.

COMPARATIVE EXAMPLE II

Example II was repeated except that the potassium permanganate solutiondid not contain sodium hypochlorite. This solution required 15.6 g KMnO₄to maintain the original permanganate concentration and the ratiodropped to 0.40. The solution had an amorphous brownish blackprecipitate at the bottom of the container.

EXAMPLE II and Comparative EXAMPLE II demonstrate the reduced level ofKMnO₄ needed to maintain the original concentration of the solution andthe lack of insoluble matter contaminating the solution, which mattermust be cleaned from the container. The solution of the invention hasincreased activity compared to the solution not containing NaOCl andlower operating temperatures and/or shorter contact times can beemployed. Further, a separate rejuvenation procedure such as shown inU.S. Pat. No. 4,042,729 is not required.

What is claimed is:
 1. In a process for treating plastics to improve the adhesion of electroless metal plating to the plastic wherein the plastic is contacted with an alkaline permanganate solution for an effective time at an elevated temperature to improve the adhesion, the improvement comprising:adding to the solution a secondary oxidant compound capable of oxidizing manganate ion to permanganate ion and controlling at desired intervals the ratio of permanganate ion concentration to the sum of the permanganate and manganate ion concentration at a level above about 0.5 by adding, as needed, an effective amount of the secondary oxidant to the solution.
 2. The process of claim 1 wherein the ratio of permanganate ion concentration to the sum of the permanganate and manganate ion concentration is controlled at a value above about 0.7.
 3. The process of claim 1 wherein the permanganate ion concentration is maintained at a predetermined level in the solution by adding permanganate ions to the solution.
 4. The process of claim 1 wherein the secondary oxidant is selected from the group consisting of chlorine, bromine, ozone, hypochlorite salts, metaperiodate salts and trichloro-s-triazinetrione salts.
 5. The process of claim 4 wherein the secondary oxidant is NaOCl.
 6. The process of claim 1 wherein the plastic to be treated is the plastic substrate of an electronic circuit board.
 7. The process of claim 6 wherein the plastic is epoxy.
 8. The process of claim 1 wherein the alkaline permanganate solution comprises:(a) permanganate salt in an amount of about 10 g/l to its limit of solubility; (b) secondary oxidant in an amount of about 1 g/l to 100 g/l; and (c) alkali metal hydroxide in an amount of about 20 g/l to 90 g/l.
 9. The process of claim 1 wherein the temperature of the solution is about 100° F. to 200° F.
 10. The process of claim 1 wherein the temperature of the solution is about 140° F. to 160° F. 